1. Field of the Invention
The present invention relates to a device and a method for separating particular image components using more than one radiographic images which are taken with radiations having different energy distributions, and a computer readable recording medium containing a program for causing a computer to carry out the method.
2. Description of the Related Art
A technique called energy subtraction has been known in the field of medical image processing. In this technique, two radiographic images of the same subject are taken by applying radiations having different energy distributions to the subject, and image signals representing corresponding pixels of these radiographic images are multiplied with suitable weighting factors and subtraction is carried out between pairs of the corresponding pixels of the images to obtain difference signals, which represent an image of a particular structure. Using this technique, a soft part image formed by removing the bone component from the inputted images or a bone part image formed by removing the soft part component from the inputted images can be generated. Using an image from which parts that are not of interest in diagnosis have been removed for image interpretation, visibility of the part of interest in the image is improved (see, for example, U.S. Pat. No. 6,421,419).
A technique called Independent Component Analysis (ICA) is known. This technique is used to separate a plurality of statistically-independent unknown component signals from a plurality of known observation signals. This technique is applied, for example, to extract voices of respective persons (component signals) from recorded contents (observation signals) which have been obtained by recording the voices of the persons through a plurality of microphones.
It is suggested that this independent component analysis technique is applicable to the above-described energy subtraction. Specifically, an absorption spectrum of a radiation varies depending on the energy level thereof and/or the component of a subject, such as soft parts or bones. That is, a radiographic image obtained by exposure to radiation is a mixture of different component images. Therefore, it is suggested that the independent component analysis technique may be applied to the energy subtraction to separate a plurality of image components (component signals), which represent soft parts and bones, from two radiographic images (observation signals) obtained with radiations having different energy distributions as described above (International Patent Publication No. WO03/073064).
In the independent component analysis technique, it is assumed that the component signals to be separated are statistically independent from each other and that the component signals contained in the observation signals are mixed at constant ratios. In practice, however, it is difficult to satisfy such preconditions, and the individual component signals may not always be restored appropriately.
Therefore, the above-mentioned International Patent Publication No. WO03/073064 proposes a partial independent component analysis approach, in which, when gene expression profiling using cDNA microarrays is carried out according to the independent component analysis, the independent component analysis is applied on a gene informative index subspace containing only differentially expressed genes, rather than using a gene informative index space (the entire observation signal) which contains genes causing separation error, to separate expression levels of the genes (component signals) supplied by malignant and stromal cells. In this approach, component signals obtained through the independent component analysis applied on the entire gene informative index space are repeatedly evaluated according to a predetermined measure to identify a gene informative index subspace, which satisfies the statistical independence condition for the component signals, from the entire observation signal.
The above-mentioned International Patent Publication No. WO03/073064, however, does not teach or suggest a problem and a solution which are inherent to the independent component analysis or the partial independent component analysis being applied to the energy subtraction carried out on radiographic images.